1. Field of the Invention
This invention relates to the field of mass air flow sensors and, in particular, to a housing unit for such a sensor.
2. Prior Art
There exists a condition in many internal combustion engines, when the air intake valve is open during the initial part of a compression stroke or prior to the completion of an exhaust stroke, which produces a momentary backflow of air through the engine's air intake manifold and mass airflow sensor.
There are known thermal mass air flow sensors such as taught in U.S. Pat. No. 4,974,455, which are not capable of distinguishing between normal air intake flow and backflow. Further, these sensors do not address the issue of particulate contamination on the mass air flow sensor element itself. Backflow within the intake manifold will result in an erroneous mass air flow signal which in turn will result in an incorrect response by the engine control systems dependent upon the signals. Particulate contamination of the air flow sensor results in readings that are not representative of the actual conditions. This is a result of the accumulation of particulates which reduce and obstruct the air flow passing by the sensing element.
Thermal mass air flow sensors of the type taught in U.S. Pat. No. 4,215,565, U.S. Pat. No. 3,352,154, and U.S. Pat. No. 4,434,656 acknowledge the issue of particulate contamination. The first two aforementioned patents, while acknowledging the issue of contamination, do not demonstrate a device which may effectively prevent it. U.S. Pat. No. 4,434,656 teaches a sensor system whereby the sensor is located along side a bluff body significantly larger than the diameter of the sensor wire such that the wire is neither in the stagnation zone where the particles are trapped or in the turbulent zone where faulty readings will occur due to turbulence. This device although adequately addressing the issue of particulate contamination, does not resolve the issue of faulty readings due to backflow over the sensing element.
Thermal mass air flow sensor taught in U.S. patent application Ser. No. 08/043,719, filed on Apr. 4, 1993, on behalf of inventors L. A. Zurek and L. J. Clowater, addresses the issue of backflow within the air intake system. The device includes a main air passage which leads to the engine, and an auxiliary passage which runs parallel to the main passage and leads into a recovery chamber, which subsequently is joined with the main air passage further downstream. Results for this device indicate significant improvement with respect to backflow induced faulty readings. However, this device does not accommodate the requirement of a contaminant-free sensor. These are some of the problems this invention overcomes.